Currently, various fastener impacting tools are powered by electricity, thus also referred to as DC electric tools. The current DC electric tools mostly have an adjustable function for the torque output. The current DC electric tools may have three torque ranges, i.e., high, middle and low torque ranges, which can be switched therebetween by an adjusting switch. In each range, the torque output can be adjusted between zero and the maximum value in this range by a trigger, for example via use of pulse width modulation or PWM, but the decreasing of the speed and the output torque due to the decreasing of the voltage of the battery in the DC electric tool is not taken into account, and the tool is simply set with a plurality of levels to speed. When the voltage of the battery is decreased, the decreasing of the voltage of the battery may cause the decreasing of the maximum speed or the torque output, but the users cannot detect it, which causes the users to not be able to select the suitable level for the current operating condition or the fastener cannot be fastened tightly, thereby affecting the normal and reliable operation of the DC electric tool.